Distinct circulation states of the Arctic atmosphere induced by natural climate variability

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Abstract
The high‐resolution regional climate model HIRHAM4 with a horizontal resolution of 0.5°×0.5° was used to determine the influence of natural climate variations in the Arctic. The large‐scale lateral forcing and the lower boundary forcing for the HIRHAM4 were delivered from a long‐term run of the coupled circulation model ECHO‐G at T30 resolution. This run was carried out with prescribed constant external forcing for present‐day climate conditions. 600 years of this ECHO‐G simulation were analyzed to find periods persisting over several years either with mainly warm or mainly cold conditions during the Arctic winter. Two warm and two cold periods, each of 6 years duration, were selected for the regionalization of the Arctic climate in January. The performed model simulations show that a warm or cold Arctic winter climate is connected with two distinct circulation states of the Arctic atmosphere. These states are characterized by a different location and extension of the tropospheric vortex with a vortex center over the western Arctic in warm Januaries or over the eastern Arctic in cold Januaries, respectively. Moreover, there are indications that these different locations of the vortex are linked to a different synoptic activity in Alaska and the eastern part of the Arctic which is, in turn, related to a different meridional transport of heat and moisture into these regions. The resultant climate conditions in these regions differ from each other with statistical significance and are, finally, decisive for the warm or cold Arctic climate state.

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